The present invention relates to a vehicle brake system with a hydraulically controllable disc brake device, which has an electromechanical actuating device for activating a parking brake function, the vehicle brake system having a hydraulic circuit with a hydraulic pressure source, as well as controllable hydraulic function elements, in order to hydraulically control the disc brake device in accordance with a service brake action of a driver or an automatic activation, the vehicle brake system further having a control device, in order to control the electromechanical actuating device in accordance with a parking brake action of the driver or an automatic activation of the parking brake function. The invention furthermore relates to a method for controlling a parking brake system of the kind described above.
Vehicle brake systems are known in diverse forms from the prior art. Specifically, the document DE 10 2005 051 082A1, and corresponding U.S. Pat. No. 8,322,495B2, both of which are incorporated by reference herein in entirety, describes a vehicle brake system with a disc brake designed as a floating-caliper disc brake. In this system, a friction pad arrangement with two opposite friction pads on either side of a brake disc is provided. One friction pad lies against a floating caliper, whereas the other friction pad can be hydraulically displaced relative to the floating caliper via an actuating piston. As a result of a hydraulic charging of an actuating piston, the movable brake pad is displaced onto the brake disc. The brake pad fixed to the floating caliper is drawn in a manner known per se, via the floating-caliper function, from the other side against the brake disk, so that a braking force acts on the brake disc on both sides.
In addition to this floating-caliper function which has been known for a long time, however, this prior art moreover also provides an electromechanical actuating device. This device is used to perform a parking brake function. With corresponding control, besides the hydraulic displacement, the disc brake can also be electromechanically actuated as well. The electromechanical actuating device is usually provided with self-locking, so that when the parking brake function has been activated and the brake tensioned, the system remains in the parking brake state and is not independently released therefrom. This ensures that this state is maintained after parking the vehicle and when the parking brake is activated. To release the parking brake function, the self-locking must then be overcome.
However, it has been found that on releasing the parking brake function, problems may arise if the self-locking is to be overcome by means of the electromechanical actuating device. This is due to the fact that the parking brake function is released jerkily or suddenly when the required actuating force for overcoming the self-locking is reached or exceeded. This is felt by the driver as abrupt releasing of the parking brake, which may be felt as a loss of comfort during starting processes, in particular when the starting process takes place downhill. In the event of an abrupt release of the parking brake, there results a correspondingly high acceleration, which is intensified by the downhill force. In order to avoid this, the driver would have to intervene with an active braking process.